In its simplest form, it’s a keto friendly concoction of good coffee, MCT oil and grass-fed butter. It doesn’t really give you much of an insulin spike, the MCT oil is quickly converted into keytones and it has healthy fat. You probably want to avoid it during a fast if you can, but it’s a nice keto friendly boost otherwise.
Q: Will it break a fast?
A: It depends on why you're fasting. It won't take you out of nutritional ketosis, but it will interrupt autophagy because you have to digest it.